Of commercially available structural materials, magnesium alloys have the smallest density and excellent specific strength and specific stiffness, and thus has been widely applied not only to parts of aircraft or automobiles, but also to parts of electronics or leisure products. Presently, almost all the magnesium alloy products have been manufactured mainly by die-casting process. However, with the demand for magnesium alloy products having various shapes and excellent properties is drastically increased, the development of new techniques for manufacturing semi-product or final product using a plastic working process, such as extruding, rolling, sheet forming, forging, etc., has been studied in recent years.
As for the casting process, an alloy is melted, supplied into a mold having a predetermined shape and then solidified to a desired product. On the other hand, in case of wrought alloy, an intermediate material, such as a billet, a slab or a plate, is prepared and plastically deformed into semi or final product. Examples of techniques for preparing an intermediate material for a plastic working process include, but are not limited to, a method of preparing a billet or slab in a batch type casting, a vertical continuous casting method for supplying a melt into a vertically disposed mold and solidifying the melt, and a horizontal continuous casting method for supplying a melt into a horizontally disposed mold and solidifying the melt.
However, the method of preparing a billet or slab in a batch type suffers because surface defects occur due to solidification shrinkage, segregation and microstructural non-uniform. Further, the properties of the intermediate material are not good and the melt loss are high. Furthermore, the productivity is decreased.
Meanwhile, in case of the vertical continuous casting method, it is difficult to prepare intermediate materials having various shapes and small cross-section area. In addition, the vertical continuous casting method is a semi-continuous casting process. Thus, the casting process should be interrupted after it has predetermined length. In contrast, the horizontal continuous casting process is advantageous because an intermediate material having good quality can be continuously prepared, and products having various shapes, such as plate-, rod- or pipe-shapes, may be easily prepared.
Although the horizontal continuous casting technique may be commercially applied to an aluminum alloy and a copper alloy, such a technique is difficult to actually apply to a magnesium alloy due to relatively lower flowability and higher reactivity with oxygen of the magnesium alloy, compared to those of the aluminum or copper alloy. In particular, when the magnesium alloy melt comes into contact with water, a sudden explosion may occur. Therefore, a continuous casting process must be developed in consideration of safety hazards.
In this regard, a horizontal continuous apparatus (U.S. Pat. No. 5,915,455), developed by Norsk Hydro, has been proposed, in which magnesium is melted in a melting furnace, fed into a holding furnace and then supplied into a mold through a melt inlet positioned at the lower portion of the holding furnace, and solidified to billet by cooling system. The horizontal continuous apparatus is suitable for the preparation of a billet or slab in which the cross-sectional area of a cast material is larger than that of the melt inlet. Further, in order to cool the melt, the cooling process, including an indirect first-cooling and direct second-cooling process has been adopted.
However, the direct cooling process used to increase the casting speed is disadvantageous because the cooling water sprayed onto the surface of the billet flows backward into the mold along the surface of the billet and thus may undesirably react with the magnesium alloy melt, or the splashed melt may react with the cooling water in the water bath, therefore sudden explosions may be generated. In practice, such accidents have been reported.